Laser diodes are typically used in optical transceivers to convert electric current into optical power for data transmission. The laser diode translates the laser current to optical power values P1 and P0, which represent the binary values “1” and “0”, respectively. Due to temperature changes and/or laser diode aging, the characteristics of a laser diode in operation will change.
FIG. 1 is a graph that illustrates the temperature dependency of a laser diode transfer function of output optical power P on the vertical axis vs. laser drive current idd on the horizontal axis. As shown in FIG. 1, when the temperature increases from temperature T0 (e.g. 25° C.) to temperature T1 (e.g. 30° C.) the optical power values P1 and P0 decrease to P′1 and P′0. Consequently, the average optical power Pave decreases as well. These variations in the average optical power and the extinction ratio P1/P0 during data transmission can reduce the reliability of a digital communication system. For example, such variations can increase the bit error rate (BER) and clock jitter at the receiver end. Also, if the original transmission is set at P1′ and P′0, and the laser temperature is decreased, the transmission power will increase, thereby overdriving the laser diode, which can damage the laser diode, as well as increasing the BER at the receiver. Additionally, as the laser diode ages its ability to efficiently convert electrical power to optical power will decrease causing the extinction ratio P1/P0 and the average power Pave to change.
To maintain a constant average optical power Pave and extinction ratio P1/P0 over a wide range of operating temperatures and over a long period of time, a laser drive current idd comprising laser bias current ib and a modulation current im is preferably adjusted to compensate for changes in the characteristics of the laser diode due to temperature changes and aging. The laser bias current drives the laser diode to a direct current operating point. The modulation current provides a switching current which varies the input data signal and has an amplitude that produces a prescribed peak-to-peak variation in the optical output power of the laser diode. As shown in FIG. 1, the binary digit “0” is transmitted if the laser drive current idd=ib(0) mA at temperature T0, while the binary digit “1” is transmitted if the laser drive current idd=ib(0)+im(0) mA at temperature T0, where ib(0) and im(0) are the laser bias current and the modulation current, respectively. As further illustrated in FIG. 1, the correct bias current ib and modulation current im for temperature T1 should be ib(1) and im(1), rather than ib(0) and im(0), to maintain the same extinction ratio P1/P0 and average optical power Pave.
There are three conventional approaches to controlling the bias current ib and the modulation current im of a laser diode to maintain a constant average power Pave and extinction ratio P1/P0.
The first approach is based on a model of linearized laser characteristics. In this approach, the bias current ib is adjusted while maintaining a constant modulation current im, until the average optical power Pave is equal to a predefined value Pref. Pref is the reference average output power from the laser diode at the desired P1 and P0 levels. Pref=(P1+P0)/2. Pref is the mean of Pave. The modulation current im is then adjusted while measuring the slope efficiency K, which is defined as the change in power P over the change in laser drive current idd (ΔP/Δidd). The modulation current im is adjusted until ΔP is equal to a predefined ΔPref.
FIG. 2 is a graph illustrating a linearized laser diode transfer function. Since ΔP/Δim=(P1−P0)/im, for a given ΔPref (e.g. ΔPref=0.05*(P1 −Po)), if ΔP<ΔPref, for example, then Δim should be increased, correspondingly since Δim=0.05 im, so im increases as well. Since Δim increased, ΔP will be larger, until ΔP=ΔPref. At this point, im=desired im, and the extinction ratio P1/P0 as well as the average power Pave is set to the desired level. Especially for higher temperatures, the method described above often yield a higher extinction ratio then the desired level because the power-current (P-I) characteristics of a practical laser diode are nonlinear.
FIG. 3 is a block diagram of a conventional system using an automatic power control (APC) loop that can be used in the first approach to implement the control loop described with respect to FIG. 2. FIG. 3 comprises a monitor photodiode (MPD) module 404 including a photodiode 407, a laser diode (LD) 402, and an automatic power control (APC) control circuit 310. The photodiode 407 is preferably coupled back-to-back and closely spaced apart from the laser diode (LD) 402 so that it receives a portion of the output optical power emitted from the LD 402. The MPD module 404 converts the optical output power into electric current ip having a proportional relationship to the optical output power. FIG. 3 also provides an illustrative context for a second approach for adjusting the modulation current 1m based on information extracted from the variation of the measured current ip. There are various methods for estimation of the optical signal extinction ratio in this approach. For example, detecting the current peak level can be used or using a square-law portion of the transfer function of an RF diode can be used to process the measured MPD current for extinction ratio adjustment.
A third approach uses a look-up table (LUT) based on temperature reading to adjust ib and im. This approach, however, is labor intensive, due to the requirement to measure the laser diode P-I characteristics device by device. This approach can also provide inaccurate adjustments to the extinction ratio P1/P0 because the reading from LUT will not be accurate if the LD characteristics of the laser diode change over time, for example, in case of LD aging.
The three conventional approaches described above are either too labor intensive (e.g., the LUT approach) or fail to meet restrictive requirements in some applications in which the variation of the average power and extinction ratio are limited within a fractional dB of the required targeted level over a wide range of temperature variations (e.g., in the range of −45° C. to 85° C.).
Accordingly, there is a need for an improved technique for maintaining a desired average power Pave and extinction ratio P1/P0 of a laser diode over a wide range of temperature variations and through device aging, while eliminating the labor intensive measurements associated with conventional LUT techniques.